Stone & Limestone Restoration

Chicago has a great deal of limestone: greystone two-flats, limestone sills and lintels on brick bungalows, stone trim and water tables on prewar homes. It is beautiful material and it is surprisingly easy to damage with the wrong repair.

Softer than it looks

Limestone is calcium carbonate. It is porous, it is relatively soft, and it reacts with acid. That means two things matter enormously.

First, the mortar has to be softer than the stone. A lime-based mortar lets the wall move and takes the wear itself. Portland-heavy modern mortar is harder than the limestone around it, so the stone becomes the weakest point and the joint edges start spalling.

Second, cleaning has to be gentle. Sandblasting removes the dense outer skin that limestone forms over time, leaving the soft interior exposed. A lot of the badly deteriorated greystone facades around the city were blasted in the 1970s and 80s and have been deteriorating faster ever since.

Common repairs

• Sills. Limestone sills take standing water and salt. They crack, they spall at the front edge, and they lose their pitch so water runs back toward the window instead of away. Many can be patched or given a dutchman repair rather than replaced.
• Dutchman repairs. A section of matching stone cut and let into the damaged area, pinned and set. Done well it is nearly invisible and lasts as long as the original.
• Patching. Color-matched, lime-based composite patching for smaller losses, mixed to match the stone rather than a gray Portland smear.
• Repointing. Open joints raked out and repointed in an appropriate lime mortar, tooled to match.

Cleaning

The right approach is the gentlest method that achieves the result: low-pressure water, appropriate detergents, poultices where staining is deep. Slower than blasting, and it is the difference between a facade that lasts another century and one that needs major work in fifteen years.